Multi-core transformer plasma source

ABSTRACT

A transformer-coupled plasma source using toroidal cores forms a plasma with a high-density of ions along the center axis of the torus. In one embodiment, cores of a plasma generator are stacked in a vertical alignment to enhance the directionality of the plasma and generation efficiency. In another embodiment, cores are arranged in a lateral array into a plasma generating plate that can be scaled to accommodate substrates of various sizes, including very large substrates. The symmetry of the plasma attained allows simultaneous processing of two substrates, one on either side of the plasma generator.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/839,360, entitled “MULTI-CORE TRANSFORMER PLASMA SOURCE,” filed Apr.20, 2001 now U.S. Pat. No. 6,755,150, the entire disclosure of which isincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Plasmas are used for a variety of purposes in the fabrication ofsemiconductor devices, such as integrated circuits, and other types ofsubstrates, such as micro-electro-mechanical (“MEMs”) substrates toachieve a variety of results. Plasma methods include the formation of alayer using plasma-enhanced chemical vapor deposition and etchingtechniques, such as reactive ion etching. A plasma might also be used toclean a processing chamber, or to prepare a surface of a substrate for asubsequent process step, such as a plasma wafer surface clean oractivation prior to formation of a layer on the surface. Plasmagenerators are also used as a source of ions for ion implantation or ionmilling. A directed plasma might also be used as a plasma torch forcutting applications.

The wide application of plasma processing has resulted in a wide varietyof plasma processing systems and apparatus. One type of plasmaprocessing chamber places the wafer on an electrode of the plasmacircuit, opposite another planar electrode, and capacitively coupleshigh-frequency electrical power to the two electrodes to form a plasmabetween them. Such a plasma reactor has advantages where it is desirableto form the plasma in the presence of the substrate, such as when thephysical movement of plasma species to and from the substrate isdesired. However, some devices or materials might not be compatible withthis type of plasma formation, particularly the bombardment by plasmaspecies, including high-energy photons, and associated heating of thesubstrate.

Another approach to plasma processing generates plasma in a remotelocation, and couples the plasma to a processing chamber. Various typesof plasma generators have been developed, including magnetron sourcescoupled to a cavity, inductively coupled toroidal sources, microwaveirradiation directed at a plasma precursor, electron-cyclotron resonancegenerators, and others. Remote plasma techniques offer a number ofadvantages for certain types of processes, such as cleaning depositionchambers, but generally the plasma that eventually reaches the chamberis of relatively low density, due to recombination of the reactiveplasma species with each other or with components of the processingsystem, such as the chamber walls or delivery conduit.

Inductively coupled plasma systems have been developed that can generatea high-density plasma in one portion of the processing chamber (e.g.above the wafer), yet shield the wafer from the more deleterious effectsof the plasma generation process by using the plasma itself as a bufferbetween the wafer and the plasma generation region and typically relieson diffusion of plasma to provide a uniform ion density across the wafersurface. In one system, a dielectric dome, or chamber top, has aconductive coil wound around the dome. High-frequency electric energyprovided to the coil couples to a plasma precursor gas in the chamberand converts the precursor to plasma. In some systems, a second powersupply couples an alternating field to the wafer or wafer supportstructure, and allows a directional component to and from the wafer tobe added to the plasma generated by the coils. Such systems are used forboth deposition and etch processes to achieve very desirable results,generally providing both high rates and good uniformity across a wafer.

However, the fields generated by the coil through the dome have anelectric field component normal to the surface of the dome that causesplasma species to be directed to and from the inner surface of the dome.This field component acting on the plasma can cause physical erosion(“sputtering”) of the inside of the dome, as well as affect the powercoupling to the plasma, thus causing a non-uniform plasma density. Insome instances the plasma might contain species that react with thematerial of the dome, further eroding the dome and potentially creatingparticles than can fall from the dome onto the wafer, creating defects.Reaction of the dome material with the plasma often arises in an etchprocess when the material being etched is similar to the material of thedome, e.g. silica-based glass. If erosion of the inner surface of thedome continues to a point where particulate contamination or strength ofthe dome is an issue, the dome might have to be replaced, affectingthroughput of the plasma system, and potentially disrupting the productflow through an entire fabrication line.

Transformer plasma sources have also been developed using a toroidalcore. The core is typically a ferrite or similar high-permeabilitymaterial, and the plasma source acts generally like analternating-current (“AC”) transformer. Primary windings are woundaround the core and an induced plasma flux around the core acts like asecondary winding(s), the plasma flux providing a secondary current tooppose the magnetic fields in the core. In one system, a tube structureforms a continuous closed path (“loop”) that includes a leg through acenter opening of the core for transformer-coupled plasma. Another legincludes a gas inlet, and the same or another leg provides a plasma/gasoutlet. In another embodiment, one leg of the plasma loop includes thegas inlet, gas/plasma outlet, and a process wafer. Plasma formed in theloop is carried past the wafer surface by the gas flow from the inlet tothe outlet.

However, recombination of plasma species on the surface of the tubes orin the gas/plasma mixture can reduce the effectiveness of a plasmasource. Recombination generally occurs to a greater degree where thedistance between the plasma core, where the fields that generate theplasma are generally higher, to the process chamber are greater.Recombination can also affect plasma density, as can dilution with aprocess gas stream. When performing a plasma or plasma-assisted processon a wafer surface it is generally desirable to have a uniform plasma sothat the surface of the wafer is uniformly processed. Uniformityproblems are generally greater with larger-sized wafers.

Thus, it is desirable to provide a plasma system that avoids the surfaceerosion problem of conventional systems while creating a high-density,uniform plasma.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a plasma processingapparatus applicable to deposition, etch, cleaning processes, ionimplantation, ion milling, and plasma torch applications. Such processesmay be applied to a substrate, such as a silicon wafer, composite wafer,glass panel, or other materials. In some instances, the plasma generatedby the apparatus might be used for chamber cleaning purposes, in theabsence of a substrate.

A multi-core plasma source forms a number of poloidal plasma currents.In some embodiments, the cores are essentially parallel to each other,i.e. the center axis of the core tori are essentially parallel to eachother in a “flat” configuration. In other embodiments, the cores are ina series, or “stacked” configuration. In one flat configuration, anumber of relatively small plasma-generating transformer cores arearrayed across a double-walled panel. The panel has a number of throughholes, some surrounded by transformer cores, and some providing a returnpath for the plasma generated by the cores. The panel provides a uniformplasma across a relatively large surface area, and can be scaled tolarger sizes. Similarly, plasma uniformity can be improved by increasingthe number of cores and through holes. The multi-core panel can bedriven by a variety of AC, radio-frequency (“RF”), or microwave (“MW”)sources. The transformers efficiently generate plasma from a variety ofprecursors over a wide range of pressures. In another embodiment, thepanel is curved, rather than flat.

In another embodiment, two substrates are simultaneously processed in aplasma chamber using the symmetry achieved by toroidal plasmagenerators. A plasma processing system includes two substrate supportstructures that each hold a substrate facing each other with atransformer-coupled plasma generator between them.

In yet other embodiments, various configuration of transformer-coupledplasma generators are provided using multiple cores. In some embodimentsthe multiple cores promote conversion of the precursor into plasma byproviding additional plasma generating zones. In other embodiments, theplasma produced by the cores achieves a higher directionality byaligning the cores in a vertical stack. In some embodiments the plasmagenerators are external to a processing chamber, being coupled to theprocessing chamber with a conduit, and in other embodiments theprocessing chamber completes a current path for the secondary circuit ofthe transformer-coupled plasma generator.

In yet another embodiment, an ion source for an ion implantation systemutilizes the directional nature of the ion distribution along thecenterline of the toroidal plasma generators by ejecting a portion ofthe ions produced toward extraction electrodes. This is believed toallow optimizing extraction gradients for mass/charge analyzerperformance while providing a high ion flux for implantation.

In yet another embodiment, a toroidal plasma generator is placed in aplasma torch head. The plasma generator is encased within an innernozzle, thus protecting the operator from electrical shock hazard. Thepoloidal current flow minimizes erosion of the inner nozzle material. Itis believed that the toroidal plasma generator will produce plasma froma wide variety of precursors over wider pressure ranges and flow ratesthan conventional arc-discharge plasma generators.

In yet another embodiment, an ion source for an ion milling systemutilizes the directional nature of the ion distribution along thecenterline of the toroidal plasma generators by ejecting a portion ofthe ions produced toward accelerator plates. It is believed that thetransformer-coupled toroidal plasma generator will provide a high fluxof ions and that the high-density nature of the plasma along thecenterline will improve the performance of the ion milling system.

These and other embodiments of the present invention, as well as itsadvantages and features, are described in more detail in conjunctionwith the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified diagram of a plasma processing system with amulti-core plasma generator plate according to an embodiment of thepresent invention;

FIG. 1B is a simplified top view of a multi-core plasma generator plateaccording to an embodiment of the present invention;

FIG. 1C is a simplified cross section of a portion of the multi-coreplasma generator plate shown in FIG. 1B;

FIG. 1D is a simplified top view of a portion of the multi-core plasmagenerator plate shown in FIG. 1B with a surface of the plate removed toillustrate internal details of the plate;

FIG. 1E is a simplified flow chart of a method of processing a substrateaccording to an embodiment of the present invention;

FIG. 2A is a simplified diagram of a multi-wafer processing system withan internal toroidal plasma source according to another embodiment ofthe present invention;

FIG. 2B is a simplified flow chart of a method of multi-wafer plasmaprocessing according to an embodiment of the present invention;

FIG. 3A is a simplified diagram of a plasma processing system with amulti-core plasma generator according to another embodiment of thepresent invention;

FIG. 3B is a simplified schematic representation of a plasma processingsystem with a multi-core plasma generator according to anotherembodiment of the present invention;

FIG. 3C is a simplified schematic representation of a plasma processingsystem with a multi-core plasma generator according to yet anotherembodiment of the present invention;

FIG. 3D is a simplified schematic representation of a plasma processingsystem with a multi-core plasma generator according to anotherembodiment of the present invention;

FIG. 3E is a simplified schematic representation of a plasma processingsystem with a multi-core plasma generator according to yet anotherembodiment of the present invention;

FIG. 3F is a simplified schematic representation of a multi-core plasmaprocessing system with a planar array of exterior transformer cores;

FIG. 3G is a simplified perspective view of an example of a top plate ofa multi-core plasma system as could be used in the system illustrated inFIG. 3F;

FIG. 4A is a simplified diagram of a multi-core plasma generatoraccording to an embodiment of the present invention;

FIG. 4B is a simplified sectioned perspective view of a multi-coreplasma generator according to another embodiment of the presentinvention;

FIG. 5A is a simplified cross section of a toroidal transformer-coupledplasma generator;

FIG. 5B is a simplified graph of ion density versus radial distance forthe toroidal transformer-coupled plasma generator illustrated in FIG.5A;

FIG. 6A is a simplified diagram of an ion implantation system with atoroidal plasma source according to an embodiment of the presentinvention;

FIG. 6B is a simplified flow chart of an ion implantation processaccording to an embodiment of the present invention;

FIG. 7A is a simplified sectioned perspective view of a portion of aplasma torch head according to an embodiment of the present invention;

FIG. 7B is a simplified flow chart of a plasma cutting method accordingto an embodiment of the present invention;

FIG. 8A is a simplified diagram of an ion milling system with a toroidalplasma source according to an embodiment of the present invention; and

FIG. 8B is a simplified flow chart of an ion milling process accordingto an embodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

I. Introduction

Embodiments of the present invention produce a plasma from multiplecores to achieve various beneficial effects. In one embodiment, manysmall cores are used to achieve a uniform plasma over a large surfacearea. In another embodiment, multiple cores are used to provide a highplasma density with a compact, efficient plasma generator. In anotherembodiment, the directionality of the plasma is used to provide anefficient source of ions for ion implantation. In yet anotherembodiment, the temperature profile of the plasma across the center ofthe core provides a compact, efficient plasma torch. These and otheraspects of the invention will be further understood in light of thespecific embodiments discussed below and reference to the accompanyingfigures. It is understood that other embodiments may be utilized andstructural changes may be made without departing from the scope of thepresent invention.

II. Exemplary Substrate Processing System

FIG. 1A illustrates one embodiment of a plasma processing system 10suitable for a variety of plasma processes, such as plasma-enhanceddeposition processes and plasma etch processes performed on a substrateor wafer. Plasma processing system 10 includes a chamber 12 having achamber body 14 and a chamber top 16, a vacuum system 18, an alternatingcurrent (“AC”) power supply 20, such as an RF generator, is coupled to aseveral toroidal cores 21, 22, 23 by leads 24, 26 and primary coils (notshown) wound around the cores. In a preferred embodiment the toroidalcore 22 is a ferrite core, but could be other magnetic material, ormerely free space (“air”) depending on the coupling structure. As anoptional bias plasma system 30 can produce movement in the plasma normalto the surface 32 of a substrate 34 or be used to initiate a plasma.Other plasma initiators, such as a spark discharge, direct current(“DC”) electrode, or ultra-violet (“UV”) source may be used. Thesubstrate could be a silicon wafer, semiconductor-on-insulator, glasssubstrate, or other substrate for example. For ease of description, thesurface 32 will be referred to as the “process surface” of thesubstrate. It is understood that the process surface may include layersand structures previously formed on the substrate. In certainembodiments, the wafer is a silicon wafer with a nominal diameter of 200mm or 300 mm.

A gas delivery system 36 provides gas(es) to the processing chamber andother system components through gas delivery lines 38, only some ofwhich might be shown. Typical gases provided by the gas delivery system36 might include plasma precursor gases, such as a cleaning or etchingplasma precursor gas, a plasma deposition precursor gas, plasma strikinggas, plasma dilution gas, and other gases, such as a cleaning precursorgas provided to an optional remote plasma cleaning system 104, forexample. The delivery lines generally include some sort of control, suchas a mass flow controller 42 and shut-off valves (not shown). The timingand rate of flow of the various gases is controlled through a systemcontroller 44, as will be described in further detail below.

The chamber top 16 includes an outer shell 46. A plasma generator plate48 is installed between the outer shell and the chamber body 14. Theplasma generator plate has a number of through holes 52 that allow gasand plasma to pass through the plate. The magnetic field inside theferrite cores 21, 22, 23 within the plate drive the secondary plasmacurrent inside the chamber when energized by the AC power supply 20through the leads 24, 26. Primary windings (not shown) around each corecouple the AC power to the core. The primary windings can be connectedto the AC power supply in series, in parallel, or in a combination ofseries and parallel connections.

The plate can be made of dielectric materials, such as ceramic, fusedsilica, or organic polymer, or can be made primarily of metal, such asstainless steel or aluminum alloy with a dielectric gap or spacer todisrupt unwanted currents through the metal around the core. Eachtoroidal core produces an alternating poloidal plasma flow in thedirection of the arrows 25 (during one half cycle) around the corethrough the through holes, and in the other direction during the otherhalf cycle. The poloidal plasma flow is essentially piece-wise parallelto the surfaces of the generator plate, thus reducing sputtering ofthose surfaces.

In some embodiments, plasma formed by more than one core may couple inan additive or subtractive manner in a through hole, in otherembodiments, such coupling is negligible because of the distribution ofcores and through holes. For purposes of discussion, a through hole witha core surrounding it will be referred to as a generator hole, and athrough hole without a core surrounding it will be referred to as areturn hole. The return hole or holes generally completes the plasmacurrent loop around the core.

In one embodiment the toroidal cores are made of a ferrite material suchas a material sold under the trade designation “3C90” by ROYAL, PHILIPSELECTRONICS, N.V., but other ferrites or other materials, such as iron,may be appropriate, or the primary winding can define a core with adielectric material or even a void, for example.

The AC power supply is coupled to the cores by the leads 24, 26 that areelectrically connected to the primary windings around the cores. The ACpower supply could operate at a variety of frequencies, such as about400 kHz, 10 MHz, 13.5 MHz, or 60 MHz. Although two leads are shownconnecting the AC power supply to the cores, an alternative circuitconfiguration using a single lead and common ground could be used.Specifically, the power supply could be mounted directly on the chamberstructure, thus avoiding long leads to the coil and associatedelectromagnetic radiation, as well as reducing variations in loadresulting from long leads. Each core, primary coil, and generated plasmaform a transformer circuit that operates as a toroidal transformerplasma source within the interior 70 of the processing chamber when inoperation. The primary circuit of the transformer is the coil, with theplasma serving as the secondary circuit of the transformer, the primarycoupling to the secondary through the core.

As described above, the outer shell 46 and chamber body 14 can be madeof a conductive material, thus serving as a shield for electronicemissions generated by the transformer plasma sources, since eachtransformer plasma source is within the processing chamber. This notonly reduces unwanted emissions from the system, but also may allow theAC power supply 20 to operate at frequencies that would otherwisegenerate an unacceptable level of electronic noise emissions. In such anembodiment, it may be desirable to provide leads from the power supplyto the chamber that are shielded. The efficient coupling of thetransformer plasma source(s) also allow a plasma to be generated over awide range of pressure, such as from about 500 mTorr to 3 Torr, and cangenerate plasma from a wide variety of precursors.

The chamber body 14 includes a substrate support member 72, which ismounted on, and forms a continuous inner surface with, the body.Substrates are transferred into and out of chamber by a robot blade (notshown) through an insertion/removal opening (not shown) in the side ofthe chamber. Motor-controlled lift pins (not shown) are raised and thenlowered to transfer the substrate from the robot blade to the substratesupport member 72. A substrate receiving portion 74 of the substratesupport member can include a wafer hold-down apparatus, such as anelectrostatic chuck (not shown), that can selectively secure thesubstrate to the substrate support member during substrate processing,if desired. In a preferred embodiment, the substrate support member 72is made from anodized aluminum, aluminum, or aluminum oxide. Thesubstrate support member may also include a heater (not shown) to heatthe wafer during processing, or to heat portions of the chamber during acleaning process. In a preferred embodiment, the substrate supportmember holds the substrate 34 so that the processing surface 32 of thesubstrate is opposite and essentially parallel to the major plane of theplasma generator plate.

The vacuum system 18 includes a throttle body 76 that houses atwin-blade throttle valve 78 and is attached to a gate valve 80 andturbo-molecular pump 82. It should be noted that the throttle body 76offers minimum obstruction to gas flow, and allows symmetric pumping, asdescribed in co-pending, co-assigned U.S. patent application Ser. No.08/712,724 entitled SYMMETRIC CHAMBER by Ishikawa, filed Sep. 11, 1996,and which is incorporated herein by reference.

The gate valve can isolate the turbo-molecular pump from the throttlebody, and can also control chamber pressure by restricting the exhaustflow capacity when the throttle valve 78 is fully open. The arrangementof the throttle valve, gate valve, and turbo-molecular pump allowaccurate and stable control of chamber pressures from between about 1mTorr to about 3 Torr, depending on gas flow rates. It is understoodthat other types of vacuum pumps and configurations of vacuum systemscould be used with alternative embodiments of the present invention.

The AC power supply 20 operates at a nominal frequency of 400 KHz, butcould operate at different frequencies, such as 60 Hz, 2 MHz, 13.56 MHz,60 MHz, or 200 MHz, with appropriate design of the elements of theplasma system. The power supply can supply up to 8 kW, but theprocessing system typically draws about 3-5 kW when processing a 200 mmwafer. It is understood that lower or higher power levels might beappropriate according to the type of process being performed and thesize of the substrate.

A particular advantage of embodiments of the present invention utilizingmultiple ferrite cores is the relatively low quality factor (“Q”) of thetoroidal plasma generating structures (primary-core-secondary (plasmaloop)). The low Q allows a high-frequency plasma generation systemwithout the need for complicated matching circuits, as might be requiredwith higher-Q systems. The low Q also reduces the sensitivity of theplasma system to the chamber load, thus resulting in a more stable andconsistent plasma operated over a wider process range.

In a high-Q system, the power delivered to the plasma can vary as theplasma is formed or chamber conditions change. For example, a plasmamight be initiated with a plasma striker gas, such as argon. When aprecursor gas, such as NF₃ or F2, is provided to the plasma, thedissociation of the gas into plasma creates a sudden increase in plasmaspecies (pressure) as well as electrically charged particles. Thiseffect can change the load on the power supply as well as the match tothe load, resulting in reduced power transfer to the plasma andpotentially reflecting a harmful level of power back to the generator.In the present invention, a low-Q system can be implemented, avoidingthese problems.

The optional bias plasma system 30 includes a bias generator 86 and anoptional bias-matching network 88. The bias plasma system capacitivelycouples the substrate receiving portion 74, i.e. the substrate, toconductive (grounded) inner surfaces of the chamber through a commonground 90. The bias plasma system serves to enhance the transport ofplasma species (e.g. reactive ions or other particles) created by theplasma generating plate 48 to the surface 32 of the substrate.

The gas delivery system 36 provides gases from several gas sources 92,94, 96, 98 to the chamber and other system components via the gasdelivery lines 38 (only some of which might be shown). Gases can beintroduced into the chamber in a variety of fashions. For example, a topport 100 is shown as one example of a means for flowing gases in to thechamber. A gas mixing chamber (not shown) can be present between the gassources and the chamber, or the top port can be arranged with a numberof parallel or concentric gas conduits to keep various gases separateuntil reaching the chamber. In an alternate embodiment, gas conduits arepresent around the perimeter of the chamber, either above or below theplasma generating plate. In yet an alternative embodiment, a gasdelivery ring with a series of gas nozzles is provided about an innercircumference of the processing chamber. Gas generally flows from thegas inlet port(s) to the vacuum exhaust system 18. This flow can alsocarry plasma species generated by the plasma generator plate toward thesurface of the substrate. In other instances, the process wafer might beplaced close enough to the plasma generating plate that gas flow is notrequired for plasma processing of the wafer surface.

An optional remote plasma cleaning system 40 is provided to periodicallyclean deposition residues from chamber components. The cleaning systemincludes a remote microwave or RF plasma generator 106 that creates aplasma from a cleaning gas source 98 such as molecular fluorine,nitrogen trifluoride, other fluorocarbons or equivalents, in a reactorcavity 108. The reactive species resulting from this plasma are conveyedto the chamber interior through cleaning gas feed port 110 viaapplicator tube 112.

The system controller 44 controls the operation of the plasma processingsystem 10. In a preferred embodiment, the system controller includes aprocessor 114 coupled to a memory 116, such as a hard disk drive, afloppy disk drive (not shown), and a card rack (not shown). The cardrack may contain a single-board computer (SBC) (not shown), analog anddigital input/output boards (not shown), interface boards (not shown),and stepper motor controller boards (not shown). The system controlleris coupled to other parts of the processing system by control lines 118(only some of which might be shown), which may include system controlsignals from the controller and feedback signals from the system. Thesystem controller conforms to the Versa Modular European (VME) standard,which defines board, card cage, and connector dimensions and types. TheVME standard also defines the bus structure having a 16-bit data bus and24-bit address bus.

An example of a system which may incorporate some or all of thesubsystems and routines described above would be the ULTIMA™ system,manufactured by APPLIED MATERIALS, INC., of Santa Clara, Calif.,configured to practice the present invention. Further details of such asystem are disclosed in U.S. patent application Ser. No. 08/679,927,filed Jul. 15, 1996, entitled “Symmetric Tunable Inductively-CoupledHDP-CVD Reactor,” having Fred C. Redeker, Farhad Moghadam, HirogiHanawa, Tetsuya Ishikawa, Dan Maydan, Shijian Li, Brian Lue, RobertSteger, Yaxin Wang, Manus Wong and Ashok Sinha listed as co-inventors,the disclosure of which is incorporated herein by reference.

It is specifically understood that other types of chambers might beadapted to a multi-core toroidal plasma source according to the presentinvention, and that different types of wafer support systems, such as acenter pedestal, might be used, as well as different exhaustconfigurations, such as a perimeter exhaust configuration. Similarly,additional coils might be added to control the plasma density anddistribution (uniformity) inside the processing chamber. For example,instead of the metal outer shell described in conjunction with FIG. 1A,a dielectric dome or shell could be used, and a coil provided outsidethe chamber, or a coupling structure(s), such as a pole face of asolenoid, could couple to the interior of the chamber through a chamberwall. Although the plasma generator plate is illustrated as a flatplate, it could form a hemisphere or other shape.

III. Planar Multi-Core Internal Plasma Generating Plate

FIG. 1B is a simplified top view of a multi-core plasma generator plate48 according to an embodiment of the present invention. The plate may beflat, curved, or otherwise shaped. The plate includes a plurality ofholes forming conduits through the plate for the passage of gas andplasma. Within the plate are several toroidal cores with a primary coilfor each core. Some of the holes 143 pass through the center of atoroidal transformer core, while others 145 do not pass through atransformer core but provide return paths for the plasma current tocomplete the secondary circuit. Holes that pass through a transformercore can also provide a return path for the plasma current in anothertransformer circuit. In one embodiment half the holes pass through atransformer core and the other half do not. In another embodiment, onefourth of the holes pass through a core and three fourths do not. Otherratios may be selected. In one embodiment, holes around the outmost edgeare through a transformer core while center holes are through holes. Theplasma generating plate is typically intended to be placed within aprocessing chamber, such as is shown in FIG. 1A, above, and FIG. 2A,below. Hole diameter can be optimized for different processes asdetermined by mean free path and sheath thickness. In some embodiments,a larger hole diameter is used for low pressure operation and a smallerhole diameter for high pressure processes.

An AC power supply 20 provides current to the primary coils in the arraythrough leads that are coupled to the primary coils. The primary coilscan be directly attached to the leads, or one side of each coil can beconnected to a common ground with one side of the power supply. Theplate may be grounded as shown, or floating, or at a selected potential.In one embodiment, a bias voltage is applied between the plasmagenerator plate and a process wafer to provide additional control of theplasma.

FIG. 1C is a simplified cross section of a portion of the multi-coreplasma generator plate shown in FIG. 1B. Ferrite cores 21, 22 arecoupled to primary coils 147, 149, which are connected to an AC powersource (not shown in this figure). The surfaces 151, 153 of the plateare generally fabricated from a metal, such as aluminum or anodizedaluminum. Other metals can be selected according to the intendedprocess, for example, stainless steel may be used in applications wherethe risk of contamination from the elements in the steel is low. Aninsulative spacer 50 breaks the electrical path around the core(s)through the plate to disrupt eddy currents. Alternatively, the surfacesof the plate could be made of a non-conductive material. The cores arepacked in a packing material 155, such as polyamide resin or magnesiumoxide, to reduce the movement of the coils after assembly. Packingmaterial can also serve to electrically insulate the primary coils andassociated wiring from the conductive portions of the generator plate.The packing material can be applied as a powder, as a liquid, such assilicon oil, as a liquid that is then polymerized, such as polyamideresin, or can be applied in a series of steps, such as applying a sheetof durable electrical insulation on which the cores and coils areplaced, then filling in the spaces between the cores and conduits with aliquid or powder, and then applying a second sheet of durable electricalinsulation before sealing the generator plate. The generator plate mayhave additional features, such as liquid or gas cooling, which is notshown for simplicity of illustration.

FIG. 1D is a simplified top view of a portion of the plasma generatorplate shown in FIGS. 1B and 1C with a surface of the plate removed toillustrate interior details of the plate. A toroidal core 22 surrounds ahole 143 formed by the wall of the conduit 157. A primary coil 149couples electro-magnetic energy to the core 22, which couples to plasmaoutside the generator plate. The plasma flow acts as a secondary circuitto complete the transformer coupling from the AC power supply (not shownin this figure) to the plasma. Another hole 145 does not have asurrounding core, and allows plasma flow through the hole to completethe secondary circuit.

FIG. 1E is a simplified flow chart of a method of processing a substrate150 according to an embodiment of the present invention. A substrate isplaced in a processing chamber (step 152) and plasma precursor is flowninto a multi-core transformer-coupled plasma generator (step 154). Thegenerator (i.e. primary coil, core, and secondary plasma circuit path)may lie entirely within the processing chamber such as illustrated inFIGS. 1A and 2A, or may be outside of the chamber, such as illustratedin FIGS. 3A-3F. In some cases the external plasma generator is coupledto the processing chamber through a conduit, in other cases theprocessing chamber completes the secondary plasma current path aroundthe core. The plasma generator forms a plasma from the plasma precursor(step 156) to plasma process the wafer (step 158). Examples of suitableplasma processes include etching, plasma-assisted thin-film formation,and surface activation.

IV. A Multi-Wafer Plasma Processing System

FIG. 2A is a simplified diagram of a multi-wafer processing system 159with an internal toroidal plasma source 161 according to anotherembodiment of the present invention. The internal plasma source is shownas a single-core source, but a multi-core source, such as is describedin conjunction with FIGS. 1A, 1B, 1C and 1E, could be used. An internalplasma source with a single core is described in U.S. application Ser.No. 09/584,167, entitled TOROIDAL PLASMA SOURCE FOR PLASMA PROCESSING,by Cox et al., filed May 25, 2000, the disclosure of which is herebyincorporated in its entirety for all purposes. In either case, thetoroidal plasma source is a symmetrical source. That is, the plasmacurrent flows essentially equally in both directions through the centerof the torus (tori), as represented by the double-ended arrow 163.

A gas source 36 supplies gas to the process chamber 165. An AC powersupply 20 provides current to a primary coil (not shown in this figure)and the toroidal plasma source 161 forms plasma from the gas. Theexhaust system 18 removes effluent from the chamber. Two wafers 34A, 34Bopposite each other with the symmetrical plasma source in between thewafers. The plane that the toroidal core lies in (the plane ofintersection) is essentially parallel to the surface of each wafer. Thewafers are held with wafer supports 72A, 72B, which can be mechanicalsupports, such as pockets or clips, vacuum chucks, or electrostaticchucks, for example. Although the wafers are shown in a verticalorientation, other orientations are possible.

When a single torus source is employed, generally a larger diameter ofthe center opening provides better plasma uniformity. For some 200 mmwafer processing embodiments, a 10 inch or larger diameter opening isused. For some 300 mm wafer processing embodiments a 16 inch or largerdiameter opening is used. The distance between wafer and source dependson uniformity which in turn depends on gases, pressure and power. Someembodiments add a gas distribution ring to the torus to improveuniformity.

FIG. 2B is a simplified flow chart of a method 166 of simultaneouslyprocessing two wafers in processing chamber with a transformer-coupledplasma generator. A first wafer and a second wafer are placed in theprocess chamber each facing an opposite side of a transformer-coupledplasma generator between the wafers (step 167). A plasma is formed onboth sides of the transformer-coupled plasma generator (step 168) tosimultaneously process the first and second wafer (step 169). The plasmaprocess could be an etch process, a surface activation process, or aplasma-assisted layer formation process, for example.

V. External Multi-Core Plasma Systems

FIG. 3A is a simplified diagram of a plasma processing system 170 with amulti-core plasma generator according to another embodiment of thepresent invention. A gas inlet conduit 172 provides precursor gas to theplasma generator 174. A chamber exhaust 173 removes Two cores 176, 178,surround a conduit 180 of the plasma generator 174. Additional conduits182, 184, 186 complete a path around the cores for the plasma to form asecondary circuit of the transformer-coupled plasma generator. Theconduits can be made of an electric insulator, such as fused silica orceramic, or can be made from a metal, such as aluminum or anodizedaluminum if a non-conductive gap or spacer is provided in the electriccircuit around the core through the conduit walls. Primary coils 188,190 around the vertically stacked cores 176, 178, are connected to an ACpower supply 20. The primary coils are shown as being wound in-phase,that is, current flowing through each primary coil induces plasma flowaround each core in the same direction. Alternatively, the primary coilscould be wound out-of-phase to each other. While the cores are shown asbeing relatively close together, they may be separated. An insulator 192breaks the electric circuit path around the coils if the conduits aremade of a conductor. The insulator can be omitted if the conduits or onesegment of the conduit path around the core(s) is made of an insulativematerial.

FIG. 3B is a simplified schematic representation of a plasma processingsystem 196 with a multi-core plasma generator 198 according to anotherembodiment of the present invention. A gas inlet 172 and chamber exhaust173 provide gas or vapor to the plasma generator 198 and the processchamber 200. A substrate 34 is in the process chamber. It is understoodthat various types of exhaust and gas delivery systems could be used,and that the representation of the substrate is merely exemplary. Twocores 202, 204 are side-by-side. A separate conduit 206, 208 goesthrough each core. Each core has a separate AC power supply 20A, 20Bdriving the primary coils 210, 212; however, a single power supply maybe used. Using two separate power supplies allows the phase of eachtransformer to be individually adjusted by adjusting the power supply.Other adjustments, such as lead length or tuning circuits, can also beused to adjust the phase of the transformers. Insulative spacers 192A,192B break the electric path through the conduits around the cores ifthe conduits are made of metal.

FIG. 3C is a simplified schematic representation of a plasma processingsystem 214 with a multi-core plasma generator 216 according to anotherembodiment of the present invention. A gas inlet 173 provides plasmaprecursor into the conduits 181, 183, 185, and 187 and a chamber exhaust173 removes products from the reaction chamber 200. The plasma formed inthe plasma generator 216 can be used to process a substrate 34, as in alayer formation or etch process, or can be used for chamber cleaning,device cleaning, surface treating, or sterilization processes, amongothers. The AC power supply 20 provides electric current to the primarycoils 188, 190, which couple to the cores 176, 178.

FIG. 3D is a simplified schematic representation of a plasma processingsystem 218 with a multi-core transformer plasma generator 220 accordingto another embodiment of the present invention. The plasma generator hastwo cores 222, 224 with conduits 226, 228 traversing through the centerof the cores, and a bypass conduit 230 extending from the gas inlet 232to an outlet 225 providing plasma to the process chamber 200. The ACpower supply 20 drives the primary coils 227, 229 in series, but theprimary coils could driven in parallel in other embodiments. Adielectric spacer 233 in the bypass conduit provide a gap in the eddycurrent path through the conduits around both coils. A bias supply 234provides an electric potential between the substrate 34 and theconductive portions of the chamber forming a circuit through the plasmaand typically through a common ground. The bias supply can be adirect-current supply, as shown, or can be another AC supply. The biassupply can assist in the transport of ions in the plasma to the surfaceof the substrate, as with a DC bias supply, or can provide motion backand forth along a selected direction, i.e. perpendicular to the surfaceof the substrate, with an AC supply. Either type of bias supply may beadded to the systems shown in FIGS. 3A, 3B, and 3C, above. Similarly,additional cores and associated components could be added to theembodiments shown to produce additional multi-core transformer plasmagenerators.

FIG. 3E is a simplified schematic representation of a plasma processingsystem 238 with a multi-core transformer plasma generator 240 accordingto another embodiment of the present invention. In this embodiment, thegas inlet 242 provides plasma precursor(s) and other gases or vapors tothe process chamber 244. The chamber exhaust 246 creates a flow from theinlet 242 across the surface 32 of the wafer 34. The transformer cores248, 250 of the plasma generator 240 have conduits 252, 254 passingthrough the centers of the toroidal cores. A linking conduit 256 and theprocess chamber 244 complete the secondary circuit of the transformer.Dielectric spacers 258, 260 break the eddy current path around theconduit walls, which are made of anodized aluminum.

FIG. 3F is a simplified diagram of a plasma processing system 262 with amulti-core plasma generator 264 according to another embodiment of thepresent invention. A chamber lid 266 is made from a plate 268 and tubes270, 272, 274. The lid can be made from stainless steel or aluminumalloy, for example. Toroidal transformer cores 276, 278 surround theouter tubes 270, 274, typically resting on the plate 268 with anintervening spacer (not shown). In one embodiment, the outer tubes areapproximately 25.4 mm (1 inch) in diameter. A dielectric spacer 280 inthe center tube 272 breaks the eddy current path around the cores. As inFIG. 3E, above, the gas inlet 242 and chamber exhaust 246 create a flowacross the surface of the substrate 34. Alternatively, the gas or vaporcould be admitted into the processing chamber 244 from vents in thechamber lid, or vents around the perimeter of the processing chamber.Similarly, the exhaust could draw from beneath the substrate, or fromthe perimeter of the substrate.

FIG. 3G is a simplified perspective view from the top of the lid 266illustrated in FIG. 3F. Two additional tubes 271, 273 join the two tubes270, 274 shown in FIG. 3F at the center tube 272. There are alsotoroidal transformer cores 275, 279 around the base of these tubes 271,273. Leads 290, 291 from an AC power supply 20 are connected to theprimary coils 281, 282, 283, 284 around the toroidal transformer cores.276, 277, 278, 279 in series, but could be connected in parallel orseries-parallel. Similarly, different power supplies could be used todrive the various transformer circuits.

VI. Multi-Core Plasma Generators

FIG. 4A is a simplified diagram of a multi-core plasma generator 400according to another embodiment of the present invention. A gas inlet401 provides gas and/or vapor from a gas delivery system (not shown),and plasma flows out the outlet 402. It is understood that gas or vaporcan also flow out the outlet, and that additional inlets could beprovided. For example, it may be desirable to provide an additionalinlet near the outlet to provide a dilutent gas to reduce plasmarecombination or to increase the flow though the outlet withoutincreasing the flow through the plasma generation zones.

Toroidal transformer cores 405, 406, 407, 408 surround conduits 409,410, 411, 412 that carry gas or vapor through the center of the tori tobe disassociated (or at least partially disassociated) into plasma. AnAC power supply 20 provides current to the primary coils 413, 414, 415,416. In this example the primary circuits are driven in parallel;however, in another example they can be connected in series. Similarly,in another embodiment the cores can alternate sides of the plasmagenerator, or additional cores and associated circuitry can be added tosurround other conduit segments.

FIG. 4B is a simplified sectioned perspective view of a multi-coretransformer coupled plasma generator 440. The generator has an inlet 442for admitting plasma precursor(s) and an outlet 444 that provides plasmato a plasma process, such as a deposition chamber cleaning process.These designations are used solely for purposes of illustration and theactual flow may be reversed in some applications. The generator has anouter shell 446 surrounding each toroidal plasma generator stage andinner shells 447, 448, 449 surrounding the toroidal transformer cores450, 451, 452. The shells can be made of metal if a non-conductive gapor dielectric spacer 454, 455, 456 is included to prevent eddy currents.The dielectric spacer can be located in different locations around thecore. Webs 457 support the generator stages inside the outer shell 446of the plasma generator, while allowing gas and plasma to flow aroundeach core.

A primary coil (not shown) around each core couples electro-magneticenergy to the plasma generator. The electrical leads (not shown) aretypically lead out from the cores to outside the outer shell through thewebs. A bottom portion 458 of the inner shell 447 is shaped to promote asymmetrical flow of plasma around the inner shell. When AC current isprovided to the plasma generator under plasma-generating conditions,plasma flows back and forth through the centers of each toroidal plasmagenerating stage (i.e. each core, primary coil, and inner shell). Thetoroidal configuration of each stage produces a plasma densitydistribution that is greater in the center of the generator andgenerally extends beyond the inner shells. In other words, the toroidalplasma generator produces a plasma with directionality, specifically,with a high plasma concentration extending along the center axis of thecores. This directionality can be a desired attribute in someapplications, such as a source for ion implantation or ion milling, or aplasma torch application.

VII. Spatial Plasma Density

FIG. 5A is a simplified cross-section of a toroidal core 501 inside ashroud or cover 503 with a dielectric gap 505. An upper edge 507 of thecover forms a reference plane. A portion of the chamber wall is shown asdotted line 508. FIG. 5B is a simplified cross-section of arepresentative ion density distribution 511 along the radial distancefrom the center axis 509 of the torus in the reference plane.Alternatively, a constant ion density could be shown versus distancefrom the reference plane. Such a curve would have a similar shape. Theion density has a maximum 513 along the center axis of the torus, thatis, ions are essentially ejected outside of the torus along the centeraxis. It is believed this ion distribution arises due to crowding of theplasma within the inner circumference of the toroidal cover. The iondensity is bilaterally symmetrical about the plane of intersection withthe circumference of the torus, and has theta symmetry about the centeraxis. The ion density also generally represents the temperature of theplasma, so the temperature at the center axis is hotter than elsewhereat a similar distance from the reference plane.

The absolute ion density depends on many factors, such as the dimensionsof the transformer structure, including the inner diameter of the coverand radius of the core, the pressure, the plasma species, and the ACdrive frequency. However, it is possible to drive the transformer at asufficiently high frequency to establish an essentially steady-state iondistribution as shown. Thus, the transformer-coupled plasma generatorcan maintain an enhanced ion density or temperature above the referenceplane formed by the upper edge of the cover. If a more uniform plasma isdesired, the inner diameter of the transformer structure can beincreased relative to the diameter of the core. Additional plasmashaping can be done with shaped cores or core covers, or by usingelectromagnetic fields.

VIII. Ion Implantation Source

FIG. 6A is a simplified diagram of an ion implantation system 600according to another embodiment of the present invention. The systemincludes a transformer-coupled ion source 602, which is driven by an ACpower supply 20. A gas delivery system 603 provides the precursor gas orvapor to the ion source 602. The

In a conventional ion implantation system, a hot filament or arcdischarge is typically used to ionize a gas into ions for implantation.For example, the gas may provide boron or arsenic ions for P-type orN-type doping of a silicon wafer. The ions are extracted from the ionsource with extraction electrodes 604, 606 and slightly accelerated sothat an analyzing magnet 608 can select the desired ions according totheir mass and charge in conjunction with a resolving aperture 610. Theextraction electrodes are generally at different electric potentials andform an electric field gradient to accelerate ions of the properpolarity. The selected ions are then accelerated in an acceleration tube612 to a selected energy for implantation into the substrate or wafer34, also referred to as the target. A focusing element 614, neutral beamtrap 616, Y-axis scanner 618, and X-axis scanner 620 are a few of theother elements typically present in an ion implantation system.Additional elements, such as high-voltage power supplies, controllers,additional extraction electrodes and beam traps (mass resolving slits)may also be present but are not shown for simplicity of illustration. Insome systems, the extraction electrodes 604, 606 are an integral part ofthe ion source 602. A number of vacuum pumps 622, 624, 626 can beoperated to provide a selected and differential vacuums in variousportions of the system.

The transformer-coupled ion source 602 includes a toroidal core 627 anda primary coil 629 and produces a poloidal current flow around the core627 of the transformer, represented by the double-ended arrows 628, 630.For purposes of discussion, the primary coil, transformer core, andsecondary plasma circuit will be referred to as the transformer coupledtoroidal plasma generator. Additional components, such as a cover forthe core, mounting structure to hold the generator in the ion source,and a cooling system are not shown for clarity of illustration. In otherembodiments, the ion source can be a multicore plasma generator such asthat shown in FIGS. 4A or 4B.

The plasma has theta symmetry, that is, the plasma density profile isgenerally symmetrical about the center axis of the toroidal core.However, the plasma density varies along a radial direction from thecenter axis of the toroidal core. In particular, the plasma isconcentrated through the center of the core, as shown in FIG. 5B above.

It is believed that the directionality of the plasma densitydistribution along the center axis of the torus, represented by thedouble-ended arrow 630, aids in the extraction of ions from the ionsource through the opening or aperture in the ion source. The aperturein the ion source is aligned with the center axis of the torus such thatthe center concentration of ions (ref. FIG. 5B, num. 513) is “pushed”out of the ion source. In other words, the transformer-coupled toroidalion source ejects ions out of the source toward the extractionelectrodes, rather than relying on diffusion (drift) and extractionfield intrusion into the ion source chamber 601 to remove ions formimplantation out of the ion source chamber.

Providing this initial transport of plasma from the transformer coreallows greater extraction of ions over a wider range of extractionvoltages. The extraction voltage typically affects the boundary shape ofthe ions exiting the ion source; however, the extraction voltage thatachieves the optimal source ion boundary shape is not necessarily theoptimum extraction voltage for maximum ion flux. This can result inlonger implantation times and reduced throughput.

It is believed that a toroidal transformer-coupled ion generator willproduce a higher ion flux than conventional sources for similaroperating conditions. A high ion flux may also allow greater control ofthe beam shape and provide more accurate implantation. In particular, ahigh initial ion flux out of the ion source may allow a high-dose,relatively low energy (shallow) implantation with low noise anddivergence because more ions are present in the initial beam than areneeded and only a portion of the ion beam (e.g. the center portion)might be selected for acceleration.

FIG. 6B is a simplified flow chart of an ion implantation process 650according to an embodiment of the present invention. An ion precursor isprovided to transformer-coupled toroidal plasma generator (step 652).The transformer-coupled toroidal plasma generator ionizes the ionprecursor to form a plasma with a plasma density distribution varyingalong a radial direction from a center axis of the toroidal core, theplasma density being greater near the center axis (step 654). Ejecting aportion of the plasma along the center axis toward an electric fieldgradient formed by extraction electrodes (step 656), accelerating theplasma toward a mass/charge analyzer (step 658) to select a portion ofthe ions for implantation (step 660), accelerating the portion ofselected ions to a selected implantation energy (step 662) andimplanting the selected ions into the surface of a target (step 664).

IX. Plasma Torch Head

FIG. 7A is a simplified sectioned perspective view of a plasma torchhead 700 according to another embodiment of the present invention. Theplasma torch head might be used in any of several applications, such ascutting shapes from material stock or in a die-separation process. Forexample, it may be desirable to use a plasma torch instead of a saw whenseparation micro-electro-mechanical systems (“MEMS”) dice to reduceparticle generation that can degrade performance of the MEMS dice.

The torch head 700 includes an outer nozzle 702 and an inner nozzle 704.Gas from which the plasma is formed enters from the inlet side 706 ofthe torch head and plasma and gas exit the outlet 708. The inner nozzle704 includes a toroidal core 710 of a transformer-coupled plasmagenerator. A primary coil (not shown) couples electromagnetic energyfrom an AC power supply (not shown). Additional cores and primary coilsmay be stacked along the center axis of the conduit to promote thedirectionality of the plasma. In this embodiment, the core has anessentially semi-circular cross-section 711 with a long edge 709parallel to the conduit. The leads (not shown) for the primary coil canbe lead through a web, as is described according to reference numeral457 in FIG. 4B, above. The inner nozzle also includes an upper shell712, a dielectric spacer 713, and a lower shaped portion 714. A conduit716 extends through the inner nozzle. The upper shell and the lowershaped portion are made of a suitable metal or alloy, such as analuminum alloy.

Generally speaking, a high-density plasma is formed in the conduit 716in the portion of the conduit proximate to the core 710, with the plasmacurrent return path through a bypass 718. A plasma initiator device,such as an electric arc or high-frequency parallel plate initiator maybe used in some applications to assist in the initial formation of aplasma. Once the plasma is initiated, the toroidal transformer-coupledplasma generator can maintain the plasma over a wide range of operatingconditions, such as pressure (e.g., 1 mTorr to 100 Torr) and flow rate

The bypass 718 allows for a separate gas flow that does not flow throughthe conduit 716. This gas flow can serve many purposes. It can providecooling to the inner nozzle, mass transport of the plasma out the outlet708, and can dilute the plasma to reduce recombination. In oneembodiment, a separate gas, e.g. propane or hydrogen, is flown throughthe bypass while another gas e.g. oxygen, is flown through the conduit.In another embodiment, the same gas is flown through the conduit and thebypass. Some plasma is in the bypass, as well as in the conduit, tocomplete the secondary circuit around the transformer core. The taperedshape of the outer nozzle provides an increase in velocity andconcentration of the plasma and carrier gas exiting the outlet 708. Thepoloidal flow of plasma around the core 710 provides a high-densityplasma extending along the center axis of the nozzle. This directionalaspect to the plasma operates in conjunction with the gas flow toefficiently provide plasma at the outlet 708 of the torch head 700,which lies on the centerline with the conduit.

Using a toroidal transformer-coupled plasma generator within the torchhead has several advantages over conventional arc-type plasma torchheads. First, arc-type plasma generators are typically run at severalhundred volts, which can be lethal if an operator comes in contact withthe voltage. While the arc electrodes are typically unavailable to theoperator during use, exposed powered electrodes or failures in theisolation of high voltages may present a lethal electrical shock hazard.In comparison, the electrical components of the toroidaltransformer-coupled plasma generator can be completely enclosed, andremain so even during servicing of the torch head.

Second, the AC power supply can be a simple step-up/step downtransformer and in some applications might run at the frequency of theline supply (e.g. 60 Hz).

Third, conventional arc-type electrodes are exposed to the plasma andplasma precursor, often causing electrode erosion or contamination. Theerosion of the electrodes is exacerbated by the fact that the greatesterosion typically occurs at the point of the electrode, where it isgenerally desirable to generate the high voltage gradients desired forare discharge. The toroidal transformer-coupled plasma generator hascover with a relatively high surface area surrounding the core, thusintense field lines intersecting the surface of the cover aresubstantially avoided. Similarly, the poloidal plasma flux generated bythe toroidal core runs essentially parallel to the surface of the cover,thus sputtering or similar damage to the core is substantiallyeliminated.

Fourth, while arc discharge generators are relatively sensitive topressure and flows, and may become unstable or extinguish ifappropriately stable operating conditions are not established, thetransformer-coupled plasma generator can operate over a wide range ofpressures and flow rates.

FIG. 7B is a simplified flow chart of a plasma torch cutting process 750according to an embodiment of the present invention. A plasma precursoris flown from an inlet end toward an outlet end of a plasma torch headthrough a conduit passing through a center of an inner nozzle (step752). The inner nozzle includes a toroidal plasma generator that ionizesthe precursor to form a plasma (step 754) in the center conduit. Acarrier gas is flown through an outer passageway formed between theouter surface of the inner nozzle and an inner surface of an outernozzle (step 756) to cool the inner nozzle and to assist in thetransport of plasma formed in the inner nozzle out the outlet (step758). The order given is merely exemplary, and the steps can beperformed in other orders, such as initiating carrier gas flow before orconcurrently with the flow of the plasma precursor.

X. Ion Milling Source

FIG. 8A is a simplified diagram of an ion milling system 800 with an ionsource 802 according to an embodiment of the present invention. Atoroidal transformer core 804 is contained within the ion source 806 andis driven by an AC power supply 20. A primary coil (not shown) coupleselectro-magnetic energy from the AC power supply to the transformercore. The transformer core 804 is typically housed in a shell 805supported by a web, as shown in FIG. 4B, above, and includes adielectric gap in the shell and typically a shaped portion (both notshown) to direct the plasma (ions) generated by the transformer-coupledplasma generator. Additional cores may be stacked along the center axis,as discussed in reference to FIG. 4B, above.

Accelerator plates or grids (also called vanes) 808 connected to ahigh-voltage power supply 810 via power lines 812 accelerate ionsgenerated proximate to the transformer core 804, particularly thosegenerated along the center axis of the toroidal core, toward the targetsubstrate 34 in response to a voltage gradient established between theplates. The high-voltage power supply is typically a direct-currentsupply operating at between about 300-1,500 Volts.

A focusing magnet 814 powered directed by a controller 816 with a powersupply operates as a lens to produce an ion beam of a selected diameterat a selected location on the substrate 34. The substrate is held by achuck 818, such as a vacuum or electrostatic chuck. In one embodiment,an electrostatic chuck with grooves for circulating a coolant againstthe backside of the substrate is used. Helium gas is circulated in thegrooves to thermally couple the substrate to the chuck, which is cooledwith a water-based coolant. A vacuum system provides the desired chamberpressure, typically between about 10-80 mTorr, in conjunction with gasessupplied from the gas delivery system 36 through the gas conduit(s) 38.

Gas supplied to the ion source 806 can be substantially inert, such asargon or krypton, or can be reactive, such as O₂, C₂F₅H, F₂, NF₃, CF₄,C₃F₈, or SF₆. In the first case, ion milling is achieved primarilythrough physical sputtering, while in the latter cases the ion millingmay occur through both physical and reactive ion sputtering, dependingpartially on the type of material being removed. In either case, the ionmilling system provides a directed beam of ions to the surface of thesubstrate for selective removal of material. While the beam is generallydirected with the focusing magnet, in one embodiment the substrate 34and chuck 818 can be tilted and rotated with respect to the ion source806. To avoid charge build-up on the surface of the substrate, in someembodiments a secondary plasma is formed between the surface of theplasma and the wall 820 of the processing chamber 822 using a secondaryplasma supply (AC or DC) 824. This secondary plasma over the wafersurface assists in dissipating the accumulated charge to the groundedchamber wall 820, which is typically made of aluminum or aluminum alloy.

FIG. 8B is a simplified flow chart of an ion milling process 850according to another embodiment of the present invention. An ionprecursor is provided to transformer-coupled toroidal plasma generator(step 852). The transformer-coupled toroidal plasma generator ionizesthe ion precursor to form a plasma with a plasma density distributionvarying along a radial direction from a center axis of the toroidalcore, the plasma density being greater near the center axis anddiminishing with increasing radial distance from the center axis (step854). Accelerating a portion of the plasma from the center axis toward atarget with accelerator plates (step 856), focusing the ejected plasmainto an ion beam (step 858), and directing the ion beam to a selectedportion of a target substrate (step 860) to selectively remove materialfrom the substrate.

While the invention has been described above with respect to specificstructures and process steps, it is understood that the invention is notlimited to the described embodiments. In particular, alternativeconfigurations of the cover, shape of the core, core materials, orplacement of dielectric gaps, or use of other precursors or otherprocess. For example, although embodiments have generally beenillustrated with an essentially round core, the core could be made ofstraight segments, such as in the shape of a square, rectangle, hexagon,or octagon, among others. Similarly, although embodiments have generallybeen illustrated with one or two substrates, additional substrates couldbe processes, such as by placing several substrates on the substratesupport member(s). These equivalents and alternatives are intended to beincluded within the scope of the present invention. Other variationswill be apparent to persons of skill in the art. Accordingly, it is notintended to limit the invention except as provided in the appendedclaims.

1. A plasma generator comprising: an inlet in fluid communication with afirst conduit passing through a first transformer core and with a secondconduit passing through a second transformer core, wherein the firsttransformer core and the second transformer core are located within acommon plasma generation plate; a third conduit passing through thecommon plasma generation plate, and not passing through any transformercore, in fluid communication with the first conduit to complete a firstplasma current circuit around the first transformer core and in fluidcommunication with the second conduit to complete a second plasmacurrent circuit around the second transformer core; and an outlet influid communication with at least the first conduit and the secondconduit.
 2. A substrate processing system comprising: a process chamberwith a chamber inlet; a chamber exhaust; and a transformer-coupledplasma generator having a first core, a first conduit passing throughthe first core, a second core, a second conduit passing through thesecond core, wherein the first core and the second core are locatedwithin a common plasma generation plate, and a third conduit passingthrough the common plasma generation plate, and not passing through anytransformer core, in fluid communication with the first conduit and thesecond conduit to complete a plasma current circuit path through theprocess chamber.
 3. The substrate processing system of claim 2 whereinthe third conduit is a center conduit completing a first plasma currentcircuit path around the first core through the process chamber and thefirst conduit and completing a second plasma current circuit path aroundthe second core through the process chamber and the second conduit. 4.The substrate processing system of claim 2 wherein the first conduit andthe second conduit comprise metal and further comprising a dielectricspacer in the plasma current circuit path.
 5. The substrate processingsystem of claim 2 further comprising: a fourth conduit passing through athird core; and a fifth conduit passing through a fourth core.
 6. Thesubstrate processing system of claim 5 further comprising: a firstprimary coil disposed to couple electro-magnetic energy to the firstcore; a second primary coil disposed to couple electro-magnetic energyto the second core; a third primary coil disposed to coupleelectro-magnetic energy to the third core; a fourth primary coildisposed to couple electro-magnetic energy to the fourth core, whereinthe first primary coil, the second primary coil, the third primary coil,and the forth primary coil are coupled to an AC power supply.
 7. Thesubstrate processing system of claim 6 wherein the first primary coil,the second primary coil, the third primary coil, and the fourth primarycoil are connected in series with the AC power supply.
 8. The substrateprocessing system of claim 6 wherein the first primary coil, the secondprimary coil, the third primary coil, and the fourth primary coil areconnected in parallel to the AC power supply.